Usually, for most cases where correction of radius of curvature of frets, a fretboard or a fingerboard of a stringed instrument or leveling the frets are conducted, such operation is conducted after the strings of the stringed instrument are loosened or removed.
However, adjustment conducted with the strings loosened or removed cannot be perfect, since the adjustment made is disturbed when the strings are restrung.
The present invention has been made to solve the afore-mentioned problem. An object of the present invention is to enable adjustment with the strings on, such condition being the most suitable condition for adjusting the frets, the fretboard or the fingerboard, easily, quickly and with high accuracy.